Self-baking electrode for electric arc furnaces and the like

ABSTRACT

Self-baking electrode for electric arc furnaces, particularly suited to the production of metal silicon, ferro-alloys and the like, including a containing metal casing (1), an electrodic carbonaceous mass (4,7) and a supporting structure (8) for said electrodic mass, said supporting structure being manufactured from carbon fibres and/or composite materials based on carbon fibres, which permits to obtain products, in particular metal silicon, having a high titer and a low impurity content. Said electrode is also endowed with improved mechanical characteristics.

DESCRIPTION

This invention relates to a self-braking electrode for electric arcfurnaces and the like, particularly suited to the production of metalsilicon, ferro-alloys, calcium carbide, phosphorus and the like, saidelectrode consisting of a cylindrical metal casing, of an electrodicmass contained and guided by said metal casing during its formation,solidification and baking steps, and of a reinforcing and sustainingstructure which supports said mass, said structure being such as toallow to obtain the final products, in particular metal silicon, withoutany significant addition of foreign elements.

As is known, various products of the metallurgical industry, such as forexample metal silicon, phosphorus, ferrochrome, ferromanganese, calciumcarbide and the like, are obtained by reduction of the correspondingores in an electric arc furnace. The electrodes for said furnaces mayconsist of cylindrical coal blocks of proper dimensions, which consumein consequence of oxidation and of the contact with the charge, andwhich, therefore, by means of suitable devices, are lowered as they getconsumed and are then replaced by new electrodes when their dimensionshave become, in consequence of the consumption, lesser than the minimumallowable dimensions.

Instead of being made of coal, the electrodes may consist of graphite,which is a more expensive material but exhibits a higher conductivity, ahigher mechanical resistance and a higher purity, and therefore permitsto obtain products of better quality because more pure.

Another type of electrode which too is utilized in the arc furnaces isthe one usually referred to as self-baking electrode. Such electrodeconsists of a metal cylindrical casing, usually a steel casing, equippedwith radial fins in its inside, into which casing the electrodic pasteis introduced from the top, such paste being made of a variously groundcarbonaceous material and of pitch. The electrodic paste, because of theheating due to the current flow, solidifies and bakes, thus forming anintegral body with the outer metal casing to which it is anchored bymeans of said inner fins which have just the function of supporting theelectrodic paste mass. In this case the outer metal casing and the finsconsume along with the coal. By this type of electrode, which is lessexpensive than the one consisting of coal blocks or of graphite blocks,an economic saving is attained, but there is the serious drawback ofintroducing, into the furnace reaction mass, and therefore into thefinal product, the metal, usually iron, of which the casing and the finsare made, which, as told hereinbefore, gets consumed together with heelectrodic paste during the process. Consequently, this type ofelectrode cannot be utilized for the production of high-quality andlow-purity products (which, as is known, find broader and broaderutilizations in several technological fields), such as e.g. metalsilicon with a low iron content.

A progress in respect of the conventional self-baking electrode isrepresented by an electrode, always self-baking, in which, however, theouter metal casing is not bound to and integral with the electrodic massby means of the fins, but is substantially fixed, while the electrodicmass flows in its inside as it is supported by a rod-shaped steelelement not bound to and independent of the outer casing, as isdescribed for example in Italian Pat. No. 606568. By means of this typeof electrode it is possible to obtain products of better quality, sinceonly the iron, of which said rod-shaped element is made, consumes alongwith the electrodic mass and passes into the furnace reaction mass and,consequently, into the final product.

An object of the present invention is that of providing a self-bakingelectrode for arc furnaces which avoids the introduction of foreignelements, in particular iron, into the furnace charge during thereaction and into the final product, and which permits to obtainproducts, in particular metal silicon, of high quality and with a lowimpurity content.

Another object of the present invention is that of providing aself-baking electrode endowed with high mechanical and thermalshock-resistance characteristics, capable of resisting even to intensemechanical and thermal stresses without the risk of cracks and/orbreaks, splinters and the like, which represent always solutions ofcontinuity of the electrode and which cause irregularities in thecurrent distribution with consequent inconveniences in the processtrend.

These and still other objects, which will be better apparent from thedetailed description given hereinafter, are advantageously achieved by aself-baking electrode for electric arc furnaces and the like,particularly suited to the production of metal silicon, ferroalloys,calcium carbide, phosphorus and the like, of the type consisting of:

a cylindrical metal casing, preferably of stainless steel, forcontaining and guiding the electrodic carbonaceous mass, said casingbeing equipped with devices, of the conventional type, suitable forallowing shiftings in respect of the furnace's fixed structures;

an electrodic carbonaceous mass which can flow downwards in thecontaining casing inside as a function of the consumption of theelectrode's lower end, which gradually forms by feeding, into the upperportion of the casing, electrodic paste, such electrodic paste beingthen baked in the lower portion of said containing casing because of theheating due to the passage of feeding current, thus forming theelectrode's lower end on which the arc strikes;

a supporting structure suited to sustain the weight and to allow thesliding of said electrodic carbonaceous mass in the containing casing,such structure being made, according to this invention, of carbon fibresand/or of composite materials based on carbon fibres, said structureexhibiting furthermore such a configuration as to improve the mechanicalresistance and the characteristics of compactedness and homogeneity ofthe electrodic carbonaceous mass.

The self-baking electrode forming the object of the presentinvention--thanks to the supporting structure of the electrodiccarbonaceous mass prepared from carbon fibres with exclusion of anymetal component and in particular of iron--permits to obtain productshaving quality and purity characteristics analogous with the onesobtainable with the electrodes (much more expensive, as is known)consisting of coal or graphite blocks.

In fact, while in the conventional self-baking electrodes the metalsupport of the electrodic carbonaceous mass, since it consumes togetherwith said electrodic mass, gives rise to the passage, into the furnace'sreaction mass, of the metal or metals which the support itself is madeof, usually steel (such metals passing then in turn into the finalproduct, thus polluting it), in the self-baking electrodes according tothe present invention the support of the electrodic carbonaceous mass,being composed of carbon fibres, i.e. a pure material chemicallyhomogeneous with the electrodic mass, as it consumes along with saidmass, does not give rise to the passage of foreign elements into thereaction mass and, consequently, of polluting elements into the finalproduct. For example, in the production of metal silicon by using aconventional self-baking electrode, the iron amount due to the electrodewhich goes into the metal silicon produced is equal to about 1 Kg/100 Kgof silicon, whereas using the electrode according to the presentinvention, the iron amount due to the electrode which goes into thesilicon, and which in such case is due in practice only to the ironcontained as an impurity in the initial electrodic paste, is equal toabout 0.03 Kg/100 Kg of metal silcon produced.

The structure made of carbon fibres, prepared and dimensioned accordingto the present invention, has also the function of improving themechanical, homogeneity and compactedness characteristics of theelectrodic carbonaceous mass and of the under-baking and baked portionin particular. In fact, since the tensile strength of the carbon fibresvaries from a minimum of 50,000 to 100,000 psi for the type having anamorphous structure, up to 350,000 psi for the type having apolycrystalline structure, the carbon fibre structure which supports theself-baking electrode and remains incorporated in said electrode andintimately bound thereto, represents a reinforcing element towards themechanical and thermal stresses to be born by the electrode, thussubstantially reducing the danger of breaks, cracks and the like, whichjeopardize the continuity and stability of the electrode. Furthermore,such reinforcing element, thanks to the high resistance to hightemperatures of the carbon fibre, remains unaltered and retains itseffectiveness for a higher section also in proximity of the electrodelower end, which, as is known, is subjected to very high temperaturesbecause of the arc.

The low conductivity of the carbon fibre, moreover, increases theelectric resistance of the electrode and permits so to the electriccurrent to distribute more uniformly and homogeneously in the entireelectrode mass.

The carbon fibre structure supporting the electrodic carbonaceous masscan be made in different forms, provided such forms are capable offixing in the electrode carbonaceous mass by means of links, slots,rings, knots and the like.

Said structure can be prepared, for example, according to a preferredembodiment, in the form of a continuous tubular net, substantiallyconcentric to the outer containing casing, rolled up, on the upper part,on a proper roll device having the function both of delivering, by meansof unrolling, the carbon fibre structure as the electrodic carbonaceousmass, wherein the structure is buried, comes down owing to the electrodeconsumption, and of supporting said electrodic carbonaceous mass bybearing the load thereof. Another embodiment of said structure mayconsist for example in a set of carbon fibre cables, they too wound up,in their upper part, on rolls or reels always acting as delivering andsupporting devices, said cables being provided with knots, protuberancesof any kind and the like, suited to represent an anchorage for theelectrodic carbonaceous mass.

The carbon fibre roped elements utilized for manufacturing nets, cablesand other similar supporting structures, may be also prepared by usingcarbon fibres of a different type, such as for example the type "Toreka"M40 manufactured by the company Tore K.K., consisting of fibres havingan average diameter of 7 μm and an average length of 100 mm.

Said roped elements may be also prepared from carbon fibres of the typeprecoated for example with SiO₂ or with SiC in order to increase, as isknown, their mechanical and chemical resistance characteristics. Saidroped elements, in particular in all those cases where silicon does notrepresent a foreign element or an impurity, such as for example in thecase of metal silicon, can be also prepared, according to anotherembodiment of the present invention, from carbon fibres either braidedand/or blended with fibres based on SiO₂ and/or silicon carbide.

Further structural and functional properties of the electrode accordingto the invention are apparent from the following description inconnection with the annexed drawing, which is given for merelyillustrative and not limitative purposes, in which the only FIGUREschematically represents a self-baking electrode carried out accordingto the invention.

With reference to such FIGURE, the numeral 1 indicates the cylindricalmetal casing acting as a container and as a guide, preferably made ofstainless steel. Said casing can slide, whenever necessary, by means ofdevice 2, which connect it to supporting structure 3 of the electrodicgroup.

Casing 1 is filled with the electrodic carbonaceous mass 4. The rawelectrodic paste (consisting, as is known, of a mixture of variouslyground carbonaceous substances and pitch in such proportions as to reachthe desired consistency and composition in fluid substances) is fed fromupper portion 5 of containing casing 1.

Electric current is fed through plates 6 to the electrode.

Because of the heat due to the passage of the current, the electrodicpaste bakes, and the baked electrode 7 is obtained, on the lower end ofwhich the arc strikes. The weight of the electrodic carbonaceous mass 4,consisting at its lower end by baked electrode 7 and in the upper layersby electrodic paste differently baked as a function of the distance fromthe current inlet area 6, is born, according to the present invention,by the carbon fibre supporting structure 8, which, in its lower part, isintegral with electrode 7, while its upper end is wound on rolls orreels 9.

An electrode 7 consumes, unwinding device 10 for unrolling the carbonfibre structure 8 permits to cause the carbonaceous mass to flow incontaining cylinder 1 and to provide the furnace with new electrodeportions by feeding from 5 corresponding amounts of raw electrodicpaste.

We claim:
 1. A self-baking electrode for electric arc furnacescomprising a containing and guiding metal casing, an electrodiccarbonaceous mass flowing in said containing casing, and a supportingstructure for said electrodic mass, said supporting structure beingcomposed of braided carbon fibres arranged in a manner so as to form asupporting element for said carbonaceous mass.
 2. The self-bakingelectrode for electric arc furnaces according to claim 1, wherein saidsupporting structure is prepared from roped elements consisting ofbraided carbon fibres, said roped elements being arranged in the form ofa net, and/or of cable provided with knots, protuberances or the like.3. The self-baking electrode according to claim 1, wherein said carbonfibres are precoated with SiO₂.
 4. The self-baking electrode accordingto claim 1, wherein said carbon fibres are precoated with SiC.
 5. Theself-baking electrode according to claim 2, wherein said carbon fibreroped elements are braided with roped elements consisting of SiO₂fibres.
 6. The self-baking electrode according to claim 2, wherein saidcarbon fibre roped elements are braided with roped elements consistingof SiC fibres.
 7. The self-baking electrode according to claim 1,wherein said carbon fibres have an amorphous structure.
 8. Theself-baking electrode according to claim 1, wherein said carbon fibreshave a polycrystalline structure.